1. Field of the Invention
This invention is related in general to the field of light engines and light transport couplers. In particular, it relates to coupler reflectors and to a configuration that consists of the combination of backward- and forward-reflecting surfaces designed to minimize the impact of manufacturing limitations on the radiance transferred to the target.
2. Description of the Related Art
While the invention is described herein for convenience in the context of a light-emitting-diode (LED) coupled to a conventional optical fiber, it is not intended to be so limited in its applications. As is well understood in the art, LEDs consist of an active region and a surrounding structure through which the light is radiated. Light emission from the active region is generally considered isotropic within the chip, but the spatial and angular distributions that a coupling optic must address are complicated by the surrounding structure. The various materials used in the LED structure, their refractive indices, the shape of the structure, and the surface texture are examples of factors that can affect the light distribution and the efficiency with which the light is emitted from the LED surfaces. In general, the result of all of these factors is a source that has a very broad angular emission that must be harnessed for most applications through the use of an optical coupler.
A common issue in the design of optical couplers resides in the fact that a decrease in the solid angle of emission from the coupler inherently requires some minimum increase in the output aperture size relative to the source (because of the principle of energy conservation and the related concept of “etendue”). In practice, it is difficult to design optical systems that can efficiently redirect light to the desired angular spread with the minimum increase in size—i.e., the output apertures must typically be enlarged beyond the theoretical minimum to avoid losing much of the optical power. This excess increase of the output size is seen as a decrease in radiance, or brightness, as compared to a coupler operating at the theoretical limit.
Regardless of the specific application, the development of coupler reflectors that minimize the loss of brightness as the light is propagated forward has been a very desirable objective in the art. To that end, many coupling devices have been developed to direct the light emitted by the source forward toward the aperture along the optical axis of the system. Typically, focusing optics and/or concave reflectors positioned around the source in some specific geometric configuration designed to optimize the usable energy output are used. See, for example, U.S. Pat. No. 3,995,935, No. 4,257,672, No. 4,385,800, No. 4,826,272, No. 5,860,723, and U.S. Publications No. 20030091820, No. 20040136081 and No. 20040120153. In all cases, the reflectors are designed to fold the light forward toward the aperture of the coupler device. In the case of LED sources, partially or fully ellipsoidal reflectors with the LED positioned in the vicinity of the rear focus of the ellipsoidal structure have been found to be particularly useful, especially in coupler devices for fiber-optic applications.
When such concave reflectors are used to couple an LED to an output aperture, the optimized theoretical reflector solution requires that the reflector curve extend all the way to the emitter base. However, mechanical clearance may be required for a variety of reasons, including chip tolerances, placement tolerances, and bond wire clearance. Based on these competing goals, the designer is confronted with the undesirable task of balancing the radiance transfer efficiency with mechanical clearance around the chip or chip array. Insufficient spacing around a chip or array could lead to a high rate of failure. Therefore, alternative reflector configurations have been explored with the general intent of achieving high brightness preservation without the mechanical issues of close-fit designs. As a result of this effort, it was discovered that a combination of forward and backward reflective surfaces can be used advantageously not only to facilitate the process of manufacture of LED couplers, but also to improve the as-manufactured performance of prior-art couplers.